Project Summary/Abstract Skeletal muscle wasting (atrophy) results from a number of pathological insults and poses a serious threat to human health. The mTOR Complex 1 (mTORC1) signaling pathway is a major regulator of cell growth, activated in response to nutrients and exercise to promote accretion of skeletal muscle mass. During atrophy, mTORC1 activity is inhibited which is significant because it reflects a shift of this tissue from anabolism (growth) to catabolism (wasting). Interestingly, atrophy occurring from muscle disuse results in an inability of nutrients to stimulate mTORC1 activity and cellular anabolism (anabolic resistance). This observation suggests a complex interplay between mechanical and nutritional skeletal muscle inputs, though a molecular basis for this relationship has not been resolved. Modeling muscle disuse and anabolic resistance in vitro is not a trivial task; a molecular understanding of mTORC1 regulation in skeletal muscle has not been realized due to experimental limitations of current cellular models. I have developed robust in vitro systems and animal models that will enable the cellular, biochemical, and genetic strategies necessary to uncover mechanisms of crosstalk between mechanical and nutrient inputs to mTORC1 in skeletal muscle. During the mentored phase of this award (K99), I will use these tools to 1) gain an understanding of how mechanical stress (cellular tension) and neuronal input (membrane potential) influence canonical mTORC1 signaling; 2) identify molecular-mediators of mechanical and nutritional crosstalk; and 3) identify biochemically how mTORC1 regulatory complexes are remodeled during states of atrophy. These studies will provide a mechanistic basis for mTORC1 inhibition during skeletal muscle atrophy and reveal strategies to combat loss of muscle mass during disuse. As an independent investigator (R00) I will apply the cellular systems, animal models, and knowledge generated during the mentored phase to investigate an equally important question of how mTORC1 activity is disrupted in aged muscle and contributes to sarcopenia (progressive loss of muscle mass with aging). Sarcopenia represents a significant threat to our elderly population as it underlies traumatic injuries resulting from frailty. This question reflects a logical scientific progression since many hallmarks of disuse-associated atrophy are shared with aging-associated atrophy, but it is unknown if the molecular mechanisms promoting loss of muscle mass are the same in these two states. With the strategies developed in this proposal, I will 1) determine if mechanical stress influences mTORC1 activity in aged muscle, 2) identify biochemical alterations in mTORC1 regulatory proteins in aged muscle, and 3) understand physiologically, how novel mTORC1 regulatory proteins contribute to sarcopenia. Achievement of the aims outlined in this proposal will represent a significant advancement in our understanding of signaling defects that underlie muscle atrophy and provide a wealth of scientific questions to pursue as I establish an independent career in biomedical research.